It is well known that golden colored articles such as jewelry can be produced by depositing a coating of a metal nitride and/or carbide on the surface of the article. Processes for depositing such coatings have been the subject of numerous studies and patents. For example, Pulker et al. U.S. Pat. No. 4,333,962, discloses a process wherein a metal such as titanium or zirconium is evaporated in an activated nitrogen containing atmosphere to produce a metal nitride which is deposited on a substrate to produce a wear resistant, gold colored product.
While such processes produce highly wear-resistant gold coatings, the coatings d not, unfortunately, possess the same brilliance as real gold or a gold alloys. Accordingly, it is often desirable to apply an additional coating of real gold or gold alloy on top of the hard wear-resistant coating. Nishida U.S. Pat. No. 4,252,862 discloses such a process wherein a layer of titanium nitride is first deposited on a substrate by an ion plating process using an electron beam source. A layer of gold or gold alloy is then deposited on the titanium nitride layer using a resistance heated source. Another process attempting to produce a coating having both the brilliance of gold and the wear resistance of hard coatings such as titanium nitride is disclosed in Bergman U.S. Pat. No. 4,403,014. The Bergman process produces a gold composite containing both gold and a metal nitride, carbide or boride. While an interesting concept, the atomic mixing of gold with metal nitrides, carbides or borides does not result in a simple integration of desirable properties, i.e., the gold composite does not possess both wear-resistance and the brilliance of gold. In fact, the gold composites produced by Bergman's process are not even as yellow as titanium nitride coatings. Bergman's gold composites also have low luster in comparison to real gold or gold alloys. In addition, an undesirable continuous or step-wise gradient of composition also occurs in these composites along with the presence of substantial amounts of gold-titanium intermetallic compounds which form during the growth of such composites.
Yet another approach to the problem of color and wear resistance is disclosed in Snyder U.S. Pat. No. 4,591,418. This process uses cathodic sputtering to coat an article with alternative thin layers of titanium nitride and gold alloys. The laminated coating produced thereby has at least five layers and, while interesting, the laminate has been found to delaminate in time.
This delaminating or latent adhesion problem as sometimes referred to herein is believed to be caused by the alloying ingredients in gold. These alloy ingredients, particularly copper, zinc and indium are believed to diffuse to the interface of the hard coating and gold alloy in time. Once a significant concentration of these elements builds up at the interface, the gold alloy layer peels off or delaminates from the underlying hard coating layer. Thus, articles or ornaments having such coatings will, quite obviously, have a limited shelf life. A consumer having recently purchased such a product is also likely to become quite agitated if he or she notices the gold layer peeling off his recently purchased product. The problem is also not easily solved by using a layer of pure gold i.e. gold without any alloying ingredients since pure gold rarely has the color which is desired color for a particular application. To obtain the desired color, alloying ingredients must be added to the gold to tint or adjust the color as desired. Pure gold also wears away very easily. It is also, quite obviously, more expensive than most of its alloys.